Medium flow meter

ABSTRACT

The invention relates to a device for measuring the flaw of a medium, e.g., a gaseous medium or a fluid medium, which device is based on measuring and affecting the temperature distribution of a medium flowing along, which flow sensor comprises two objects, each comprising a heating element and a temperature sensor, the measured temperature difference being kept, by way of a control loop, to the value of zero, the asymmetry of the power supply to the objects in order to comply with the aforesaid criterion, according to which the temperature difference must be zero, being representative of the value to be measured of the medium flow, taking into account parameters associated with the medium, such as its density and specific heat.

[0001] The invention presented relates to gas and/or fluid flowmeasurement techniques making use of thermal effects. Convection plays arole here: a moving medium entrains heat (energy) by way of its own heatcapacity.

[0002] A known sensor, in which use is made of said property, is theanemometer. As a rule, the embodiment of such measuring device consistsof one or more objects which are heated by a specific dissipated powerand in which the flow affects the resulting temperature of said object.Said resulting temperature is a measure of the flow. Said embodiment andmethod are referred to as “Constant Power Anemometry” or simply CPAmethod. Also, said objects may be kept at a constant temperaturedifference with respect to a reference temperature, and it may bemeasured which power to be dissipated is required for that purpose. Saidembodiment and method are referred to as “Constant TemperatureAnemometry” or simply CTA method.

[0003] Apart from this, the measuring devices based on thermal effectsmay be (mechanically or fluidally) broken down into three types:

[0004] (1) devices in which the object is entirely enclosed by themedium, as disclosed in U.S. Pat. No. 4,651,564 [R1, 1987] and in [R2,1993];

[0005] (2) devices in which the object is not on all sides in touch withthe medium, as extensively discussed in [R3, 1995]; and

[0006] (3) devices in which the object, contrary to (1), encloses themedium (the medium flows through a tubular device—the object), asdisclosed in U.S. Pat. No. 5036701 [R4, 1990].

[0007] The known “hot-wire anemometry” measuring devices are of type(1), [R5, 1995].

[0008] An example of said type also is the method and device asdisclosed in [R1, 1993], in which the power dissipated in the object iskept constant (CPA), and in which the temperature (distribution) aroundthe object referred to earlier is then considered.

HEAT BALANCE OF AN OBJECT

[0009] The invention relates to the control of the temperature ofobjects which are in thermal interaction with the environment. Tosupport the specification of the invention, the so-called heat balanceof an object is first gone into.

[0010] The behaviour of an object (having on it, e.g., a temperaturesensor and/or a heating element) in a thermal interaction with theenvironment may be studied by considering the heat balance of saidobject.

[0011] The heat balance refers to the circumstance that the increase ofthe heat Q (energy dimension) stored in the object equals the heattransmitted to it (P_(in)) or generated within the object (P_(gen)),minus the heat dissipated (P_(out)) and absorbed within the object(P_(abs)): $\begin{matrix}{\frac{Q_{obj}}{t} = {P_{in} + P_{gen} - P_{out} - P_{ebs}}} & ({F1})\end{matrix}$

[0012] For the sake of clarity of the further specification, it isassumed that the internal heat absorption (e.g., such as the one by wayof an internal chemical reaction or a phase transition) equals zero(P_(abs)=0).

[0013] The heat capacity of the object (C_(obj)) determines therelationship between the stored heat (energy) and the temperature ofsaid object (T_(obj)): $\begin{matrix}{T_{obj} = \frac{Q_{obj}}{C_{obj}}} & ({F2})\end{matrix}$

[0014] The heat balance of an object then becomes: $\begin{matrix}{{\frac{Q_{obj}}{t} = {{P_{{gen}\quad} + P_{in} - {P_{out}\quad \frac{T_{obj}}{t}}} = \frac{P_{gen} + P_{inout}}{C_{obj}}}}{{{with}\quad P_{inout}} = {P_{in} - P_{out}}}} & ({F3})\end{matrix}$

[0015] Said latter formula or differential equation for T_(obj) may bewritten in the form of an integral: $\begin{matrix}{T_{obj} = {\frac{1}{c_{obj}}{\int{\left( {P_{gen} + P_{inout}} \right){t}}}}} & ({F4})\end{matrix}$

[0016] An object is in equilibrium with the environment ifT_(obj)=constant (no longer depending on time). Therefore, the followingapplies to said object:

P _(gen) +P _(inout)=0, or P_(gen=−P) _(inout)

[0017] From this, it is obvious that, for a situation in which thetemperature of the object (T_(obj)) is constant, there exists a balancebetween the internally generated power P_(gen) and the incoming andoutgoing power P_(inout).

TEMPERATURE OF AN OBJECT

[0018] The temperature of an object which is in thermal interaction withthe environment having a flowing medium, in equilibrium has atemperature which may be expressed as follows: $\begin{matrix}{{T_{obj} - T_{m}} = \frac{P_{gen}}{A + {B\sqrt{v}}}} & ({F5})\end{matrix}$

[0019] where T_(obj) is the object temperature, T_(m) is the medium andenvironmental temperature, P_(gen) is the heat generated within theobject, A is a constant representing the heat conduction from the objectto the environment, B is a constant representing the influence of theconvection of the medium on the object temperature, and v is the flowrate of the medium.

[0020] Here, four items of interest are distinguished:

[0021] 1—For a given and fixed P_(gen), A, B and v, the (absolute)temperature of the object is directly proportional to the temperatureT_(m) of the medium.

[0022] 2—For a given and fixed A, and v, the temperature differenceT_(obj)−T_(m) is proportional to the internally generated heat P_(gen):

T _(obj) −T _(m)=constant.P _(gen)

[0023] 3—The proportionality constant is referred to as the “flowsensitivity” of the object:

T _(obj) −T _(m) G=(v).P _(gen)

[0024] 4—Only the size of the flow affects the temperature and not thesign (positive or negative direction).

BRIEF DESCRIPTION OF THE FIGURES

[0025] In FIG. 1, there is given a cross section of a flow-measuringdevice. The medium flows through a tubular device (1-1). The object O₁under consideration (1-2) is surrounded by the medium.

[0026] In FIG. 2, the local (medium) temperature around the object (withrespect to T_(m)=environmental medium temperature) is given as afunction of the position. The object was in position x=0. The dissipatedpower (P_(gen)) in the object is constant (CPA method).

[0027] In FIG. 3, the object temperature with respect to medium T_(m) isgiven as a function of the flaw having a design as shown in FIG. 1. InFIG. 3, it is shown that, with an object in the CTA mode, there can bemeasured no direction of the flow.

[0028] In FIG. 4, the local temperature around the object as a functionof the position is given for a device such as the one outlined in FIG.6. The dissipated power (P_(gen)) in the object is controlled in such amanner that the temperature difference between the object and the mediumtemperature is constant (CTA method). To realise this, it is requiredthat the medium temperature be measured with an additional temperaturesensor (for this purpose, see FIG. 6).

[0029] In FIG. 5, there is given the power P_(gen) required to controlthe object at a fixed temperature with respect to the medium. At a fixedabsolute object temperature, P_(gen) is a function of the mediumtemperature. The measuring device is outlined in FIG. 6.

[0030] In FIG. 6, there is given a cross section of a flow-measuringdevice. The medium flows through a tubular device (6-1). The object O₁under consideration (6-2) is surrounded by the medium. The mediumtemperature is measured using a sensor S₂ (6-3).

[0031] In FIG. 7, there is given a cross section of a flow-measuringdevice having two objects. The medium flows through a tubular device(7-1). Two objects O₁ (7-2) and O₂ (7-3) are surrounded by the medium.

[0032] In FIG. 8, the temperature difference between the two objects O₁(7-2) and O₂ (7-3) of FIG. 7 are outlined as a function of the flow ofthe medium.

[0033] In FIG. 9, there is given a cross section of a flow-measuringdevice. The medium flows through a tubular device (9-1). In the tube,three objects O₁ (9-2), O₂ (9-4), and O₃ (9-3) are surrounded by themedium.

[0034] In FIG. 10, there is given the temperature of the threeobjects—T₁ for O₁, T₂ for O₂, and T₃ for O₃ (always with respect to themedium temperature) —as a function of the flow of the medium. Themeasuring device is given in FIG. 9. The object O₁ (9-2) is heated witha constant power P_(gen). Object O₂ (9-4) and object O₃ (9-3) are notheated.

[0035] In FIG. 11, the temperature difference of the objects O₂ (at T₂)and O₃ (at T₃) is given as a function of the flow of the medium. ObjectO₁ is driven with a constant power (object O₁ in the CPA mode).

[0036] In FIG. 12, the temperature is indicated for three objects in ameasuring device such as the one outlined in FIG. 9. The object O₁ (9-2)is heated with such a power P_(gen) that the temperature T₁ of theobject O₁ (9-2) is kept at a constant value over that of the medium.Once again, there is required an additional sensor here which is notshown in the figure. Object O₂ (9-4) and object O₃ (9-3) are not heateddirectly.

[0037] In FIG. 13, the temperature difference between the two objects O₂(9-4) and O₁ (9-3) is shown as a function of the flow of the medium.

[0038] In FIG. 14, there is shown a block diagram of the properinvention. The invention is extensively discussed in the furtherspecification.

[0039] In FIG. 15, there is given a cross section of a flow-measuringdevice having two objects The medium flows through a tubular device(15-1). The two objects O₁ (15-2) and O₂ (15-3) are surrounded by themedium. The medium temperature may be measured using a sensor S₄ (15-4).

[0040] In FIG. 16, the temperature distribution as a function of theposition is given for a measuring device such as the one shown in FIG.15. The temperature distribution is given for four values of the flow(Flow=0, Flow=v1, Flow=v2, Flow=v3). The temperature of the two objectsO₁ (15-2) and O₂ (15-3) is kept at a temperature difference of T₁−T₂=0using a controller.

[0041] In FIG. 17, the temperature distribution as a function of theposition is given for a measuring device such as the one shown in FIG.15. The temperature distribution is given for four values of the flow(Flow=0, Flow=v1, Flow=v2, Flow=v3). The temperature of the two objectsO₁ (15-2) and O₂ (15-3) is kept at a temperature difference of T₁−T₂=0using a controller. Apart from this, the temperature of the two objectsis also kept at a constant temperature with respect to the medium usinga sensor S₃ (15-4) and a controller.

[0042] In FIG. 18, there is outlined the ratio P₁/(P₁+P₂) of the powersP₁ (P_(gen) of object 1) and P₂ (P_(gen) of object 2) which are outputby the controller to keep the temperature difference of T₁−T₂ at zero.

[0043] In FIG. 19, there is outlined the ratio (P₁−P₂)/(P₁+P₂) of thepowers P₁ (P_(gen) of object 1) and P₂ (P_(gen) of object 2) which aremade available by the controller to keep the temperature difference ofT₁−T₂ at zero.

[0044] In FIG. 20, there is outlined a measuring device having twoobjects in the medium. R₁−R₂ together form object O₁ and R₃−R₄ togetherform object O₂. There is also outlined an electronic circuit which actsas a controller to set the temperature difference of T₁−T₂ at zero (asoutlined in FIG. 14). The figure is extensively discussed hereinafter inthe specification.

[0045] In FIG. 21, there is given a second embodiment of the electriccircuit. This figure, too, is extensively discussed hereinafter in thespecification.

[0046] In FIG. 22, there is given a third embodiment of the electriccircuit. This figure, too, is extensively discussed hereinafter in thespecification.

[0047] In FIG. 23, there is given a fourth embodiment of the electriccircuit. This contains a device to have the offset as indicated in FIG.14 removed using a controller. This figure, too, is extensivelydiscussed hereinafter in the specification.

[0048] In FIG. 24, there is given a fifth embodiment of the electriccircuit. This figure, too, is extensively discussed hereinafter in thespecification.

[0049] In FIG. 25, there is given a measuring device having two objects,where the medium now flows through the objects. The objects each consistof a temperature-dependent resistance which is capable of dissipatingthe object power P_(gen) and which at the same time acts as atemperature sensor of the object in question. This figure is extensivelydiscussed hereinafter in the specification.

[0050] In FIG. 26, there is given a measuring device having two objects,where the medium now flows through the objects. The objects each consistof a resistance which is capable of dissipating the object powerP_(gen). The temperature difference between the two objects is measuredusing a series of thermocouples, or a thermopile. This figure isextensively discussed hereinafter in the specification.

[0051] In FIG. 27, there is given a special embodiment of the measuringdevice as shown in FIG. 25.

[0052] In FIG. 28, there is given a special embodiment of the inventionhaving several objects. An advantage is the built-in redundancy. Shouldone of the dissipators and/or temperature sensors fail, the controlleris capable of observing this and “switch off” the objects in question.

[0053] In FIG. 29, there is given an embodiment of a measuring devicewhere the medium flows through the object.

[0054] In FIG. 30, there is given an embodiment of a measuring devicebased on the invention where the medium flows through two objects.

[0055] In FIG. 31, there is given an embodiment of a measuring devicebased on the invention where the medium flows through a tubularconstruction in which the two aforementioned objects are located.

[0056] In FIG. 32, there is given an embodiment of a measuring devicebased on the invention where the medium flows through a tubularconstruction in which there are located the two aforementioned objectswhich are inserted into the tubular construction as a probe.

[0057] In figures FIGS. 33, 34 and 35, there is given an embodiment of ameasuring device based on this invention, in which the two objects areseparated from the medium by a thin partition The objects have a thermalinteraction with the medium.

[0058] In FIG. 36 (cross section) and FIG. 37 (plan view), there isgiven an embodiment of a flow meter based on this invention, consistingof a semiconductor structure.

[0059]FIG. 38 shows a cross section through a synthetic carrier havingon it two objects, each comprising two resistance elements.

[0060]FIG. 39a shows a photograph of an embodiment having two objects,each with two resistance elements, in conformity with the figures FIG.36 and 37.

[0061]FIG. 39b shows a photograph of an embodiment having three objects,each with one resistance element.

BACKGROUND OF THE INVENTION

[0062] Flow meters, type: X

[0063] A basic outline of this type of medium flow-measuring devices isshown in FIG. 1.

[0064] They consist of an object which is in equilibrium with theenvironment. For these types, there are used both the CPA method (FIG.3) and the CTA method (FIG. 5).

[0065] In the CPA mode, the medium temperature directly affects theobject temperature.

[0066] In the CTA mode, the medium temperature directly affects theP_(gen) to keep the absolute object temperature constant.

[0067] In either mode, it is no longer possible to extract the direction(positive or negative) of the flow from the output signal of themeasuring device

[0068] These are known drawbacks of this type of flow-measuring device.

[0069] Flow meters, type: XX

[0070] A basic outline of this type of medium flow-measuring device isshown in FIG. 6. The medium temperature is measured using an additionalsensor.

[0071] For these types, once again there are used both the CPA method(FIG. 3) and the CTA method (FIG. 5).

[0072] For type XX, a major drawback of flow meters of the type X isovercome, since the medium temperature is now known.

[0073] It still is no longer possible to extract the direction (positiveor negative) of the flow from the output signal of the measuring device.

[0074] This is a known drawback of this type of flow-measuring device.

[0075] Flow meters, type: XXX

[0076] A basic outline of this type of medium flow device is shown inFIG. 7. For this type, there are used two objects.

[0077] Now, both objects are used as outlined earlier for type X. Theuse of this type in the CPA method is known. Now, it is possible todetermine the direction of the flow. A known example of this type is the“microflown” [R6, 1995; R7, 1997]. The output signal is a function ofthe temperature difference between the two objects. A known drawback isthat the zero point of this type of measuring device strongly depends onthe medium temperature. Another known drawback is that, for higher flowvalues, the temperature difference between the two objects declinesagain.

[0078] Also, for this type, the CTA method is used by keeping object O₁(7-2) at a constant temperature difference (not equalling zero) with theother object O₂ (7-3). In this case, a drawback is that the measuringdevice now acts for flow in one direction only.

[0079] Flow meter, type: IV

[0080] There is known a flow-measuring device having three objects inthe medium, as shown in FIG. 9. An example of this type is the Lammerinkdevice [R3, 1993]. Here, use is made of the CPA method. The temperatureas a function of the position is given in FIG. 10. The temperaturedifference between the two objects O₂ (9-4) and O₃ (9-3) is outlined inFIG. 11. The output signal is a function of the temperature differencebetween two objects. A known drawback is that the zero point of thistype of measuring device strongly depends on the medium temperature,Another known drawback is that, for higher flow values, the temperaturedifference between the two objects (see FIG. 11) declines again.

[0081] The use of object O₁ (9-2) in the CTA mode is known. Thetemperature distribution as a function of the position then stronglyresembles the one shown in FIG. 4. The temperatures of the three objectsas a function of the flow is shown in FIG. 12. In FIG. 13, there isshown the temperature difference between the objects O₂ (9-4) and O₃(9-3). A known drawback is that the zero point of this type of measuringdevice strongly depends on the medium temperature.

[0082] Also known is the measuring device as described in U.S. Pat. No.4,651,564[R2, 1987]. In this device, use is made of two objects in themedium. There is made use of one heating element which is distributedamong the two objects. Apart from this, both objects have atemperature-dependent resistance for a temperature sensor. A drawback isthat it is not possible to adjust the dissipated power in both objectsindependently from one another.

[0083] There is known a measuring device as described by Van Putten [R8,R9]. Using one construction having several resistance elements, the flowin two perpendicular directions is measured.

[0084] A drawback here is that the dissipated power in the heatingelements is not independently adjustable.

[0085] There is known a method as described by Huijsing [R10] to keepthe temperature of an object constant (CTA mode). Said method may beapplied to this invention in order to adjust the temperatures of bothobjects and thereby is an example of a controller. A drawback of saidmethod is that it is coupled to a digital clock signal and that it is aso-called “synchronous” digital circuit.

INVENTION

[0086] A block diagram of the proposed invention is shown in FIG. 14. Anembodiment for the construction using two objects in a tubularconstruction is shown in FIG. 15.

[0087] The proposed invention makes use of two objects which have athermal interaction with their environment which is affected, one way oranother, by the flow.

[0088] The objects each have a heat supply using at least one actuatoror dissipator, and the objects have at least one temperature sensor formeasuring the object temperature.

[0089] Also, the proposed invention includes an analogous or digitalcontroller (proportional or not) which sees to it that, by way ofgenerating heat in the object dissipators, the temperature of theobjects is adjusted in such a manner that the temperature differencebetween both objects becomes zero.

[0090] In the further application, a dissipator refers to a heat sourcewhich converts electric power into heat. This is a functional name and afunctional concept. The function may be carried out by an “electricresistance”, but also by an “active element” such as, e.g., atransistor, a bipolar element, NPN, PNP, MOST, FET, IGBT etc., or adiode, or e.g., a Pettier element (having the property of a possiblenegative heat flow; cooling).

[0091] The objects each have a temperature sensor which is used tomeasure the object temperature. The temperature measurement, too, isrepresented as a function. The word “sensor” is a functional name. Thetemperature sensor need not necessarily be a (temperature-dependent)resistance, but may be any element having said function (there areknown, inter alia, (semiconductor) elements, such as resistances,transistors, bipolar elements, MOST, CMOST, IGBT, and othermajority/minority charge-carrier elements, such as diodes andresistances). Thermocouples may also serve as temperature sensors.

[0092] The detailed specification of the invention follows by referenceto FIG. 14. The controller generates (electric) powers P₁ and P₂. Saidelectric powers are converted by the dissipators into heat flowsP_(gen1) and P_(gen2). The objects each have a heat capacity by whichboth total heat flows ((P_(gen1)+P_(inout1)) and (P_(gen2)+P_(inout2)))are integrated into physical object temperatures T₁ and T₂. By way ofthe heat flows P_(inout1), and P_(inout2), the objects have theirthermal interaction with the environment and with the medium to bemeasured.

[0093] Both object temperatures are measured using temperature sensor1and temperature sensor2, and convert the physical temperatures intoelectric signals. By subtracting said signals from one another, there isproduced the electric signal T12. From this signal, the electric signalΔT is formed by way of an optional calibration-offset circuit. Theoffset circuit is extensively discussed hereinafter. For now, we assumecalibration offset=0.

[0094] As a result of the operation of the controller, the temperaturesof the two objects will be equalised to one another. This leads totemperature distributions in the neighbourhood of the objects as afunction of the position, as shown in FIG. 16 and in FIG. 17.

[0095] The two temperatures of the two objects T₁ and T₂ are equal (orare adjusted to become equal). For a specific flow, the heat flowsP_(gen1) and P_(gen2) (see (F5)) which are required for this purpose,differ for the two elements. In the event of a positive flow, object2experiences a flow of a medium which is already preheated by object1.The asymmetry in the two heat flows is a measure of the flow. Apart fromthe control powers P₁ and P₂, the controller also supplies an“information signal”—I_(out)—to the output. The output signalI_(out)=P₁/(P₁+P₂) is shown in FIG. 18. The output signal log(P₁−P₂)/(P₁+P₂) is shown in FIG. 19.

[0096] An advantage of the invention described is that the electricoutput signal outline in FIG. 18 (and in FIG. 19) in a firstapproximation is independent from the total power P_(tot)=P₁+P₂dissipated in both objects together.

[0097] Initially, the output signal associated with a CPA (ConstantPower Anemometry: P_(tot)=P₁+P₂=constant, ΔT=0; temperature distributionof FIG. 17) is equal to the output signal associated with CTA (ConstantTemperature Anemometry T₁,T₂=constant, ΔT=0; temperature distribution ofFIG. 17).

[0098] In a first approximation, the output signal does not depend onthe temperature of the objects, nor on the total power dissipated in theobjects, but only on the flow of said medium.

[0099] Another advantage of the invention is that, in the CTA mode, thesensitivity of the temperature sensors does not affect the output signal(I_(out)) of the controller.

[0100] A well-known problem is the systematic error in the electrictemperature-difference signal T12 (see FIG. 14). Said systematic errormay be prevented by making use of two thermocouples or a thermopile (seeFIG. 26) as a temperature(difference) sensor. Due to the nature of thethermo-couples or thermopiles, these have no systematic offset error.

[0101] Making use of this type of temperature sensor in the proposedinvention, there is obtained a “natural” zero point for theflow-measuring device, without a systematic error.

[0102] Calibration option

[0103] The control criterion is to adjust the physical temperaturedifference between the two objects to zero. By, in a calibrationprocedure, bringing the controller to a calibration mode and at the sametime equalising the two powers P₁ and P₂ to zero—P₁=0,P₂=0—the twoobjects will cool down and, after a certain time interval, assume thetemperature of the medium and the environment and thereforeautomatically become mutually equal (without active controller), so thatT₁=T₂. This will also occur in a—flowing—medium.

[0104] After the cooling-down period, so that T₁=T₂, the electric signalT12 may be measured and the “calibration offset may be made such thatthe electric signal ΔT=0. After the calibration phase, said calibrationoffset” value may be used as a compensation of the systematic error ofthe temperature sensors.

[0105] Self-analysis

[0106] Since the Controller is capable of resetting ΔT to zero, it isalso clear when this aim is not attained. The attaining, or not, of thedirectly imposed aim constitutes a criterion for the operation of theflow measurement, and this information may be used as such.

[0107] It should be noted that the output signals, as drawn in figuresFIG. 18 and FIG. 19, partly depend on various parameters associated withthe medium, such as the density of the medium, the specific heat of themedium, and the flow rate.

EMBODIMENTS

[0108] The invention includes various embodiments relating to theelectric circuit and the controller, as well as the construction of theobjects relating to the flowing medium.

[0109] First, there will be described seven electric embodiments of theinvention is with reference to FIG. 20 up to and including FIG. 26.

[0110] A generic embodiment of the invention is shown in FIG. 24. Here,two temperature/dependent resistances are used as objects. The objectsare surrounded by flowing medium. By applying the electric currentsI₁,I₂ and measuring the resulting voltages V₁,V₂, the controller knowsthe powers (P_(gen1)=I₁*V₁) and (P_(gen2)=I₂*V₂) dissipated andgenerated in the two objects. The controller also knows the temperatureof the two objects by simultaneously determining V₁/I₁=R₁ and V₂/I₂=R₂.

[0111] In the controller, there may also be implemented a calibrationprocedure with the purpose of, during said procedure, halting thecontroller, resetting the powers P₁ and P₂, detecting the systematicerror, and recording it by way of a calibration-offset value.

[0112] There is drawn a digital controller (proportional or not) havingtwo analogue/digital converters and two digital/analogue converters. Theproposed controller may also be designed in analogue form, the fourconverters referred to then being cancelled.

[0113] In FIG. 20 up to and including FIG. 23, there are givenembodiments of the invention, the temperature difference of two objectsin a medium flow being controlled. The objects always consist of acombination of two resistance elements which are in close thermalcontact with one another. The resistance R1 and the resistance R2together with their close thermal connection constitute an object whichis considered as object R1#R2, and the resistances R3 and R4 togetherconstitute an object which is considered as object R3#R4.

[0114] Due to the nature of the embodiment of the electric circuitsshown in figures FIG. 20 up to and including 23, said circuitsautomatically start to oscillate with their own specific so-calledfreewheeling frequency.

[0115] The temperature-dependent resistances R2 and R3, which act astemperature sensors of the objects R1#R and R3#R4, are always includedin a bridge circuit. The resistances have a positive temperaturecoefficient.

[0116] The resistance R1 constitutes the dissipator of the object R1#R2and the resistance R4 constitutes the dissipator of the object R3#R4. Byway of a comparator, the output signal of the aforementioned bridgecircuit is assessed, and it is determined whether object R1#R2 has ahigher or a lower temperature than object R3#R4. If object R1#R2 has ahigher temperature than object R3#R4, the output of the comparator willbe “low”, and by way of the inverter the input of electronic switch Q2will be “high” and will be opened. Due to this, resistance R4 will startto dissipate until the output signal of the bridge is inverted. At thatpoint in time, the temperature of object R3#R4 is higher than the one ofobject R1#R2. The comparator will then invert once again and the outputof the comparator will become “high”. As a result, switch Q1 will beopened and resistance R1 will start to dissipate. Q2 will be closed andresistance R4 will stop to dissipate.

[0117] The circuit described will start to oscillate with a frequencydetermined by the time constants of the objects R1#R2 and R3#R4. Thesignal shape at the position “info out” in the circuit will have theform outlined in the figure. During t₁, resistance R1 will dissipate andduring t₂, resistance R4 will dissipate. Since the two switches areidentical and the dissipation resistances are provided with the samesupply voltage, the momentary power dissipated will be equal for bothresistances at the point in time on which a resistance dissipates. As aresult, the period of time during which the switches are “on” is adirect measure of the average power dissipated in the resistances.

[0118] Herewith, the ratio of the time intervals t₁1(t₁+t₂) is equal tothe power ratio P₁/(P₁+P₂) (see FIG. 18). Also, the ratio(t₁−t₂)/(t₁+t₂) is equal to (P₁−P₂)/(P₁+P₂) (see FIG. 19).

[0119] In the embodiment shown in FIG. 21, the comparator directlycontrols the dissipator resistances R1 and R4, and the use of additionalelectric switches, such as Q1 and Q2 in FIG. 20, is avoided.

[0120] The embodiment of the invention, as outlined in FIG. 22, issubstantially similar to the embodiment of the invention as outlined inFIG. 20. Here, the use of the inverter circuit is avoided by directlycoupling the electric switch Q1 to the output of switch Q2. For therest, the functional operation of the electronic circuit in thisembodiment is equal to the one shown in FIG. 20.

[0121] The other embodiment of the invention is shown in FIG. 23. Thisembodiment includes a (digital) controller circuit. By way of the“check” signal, the controller controls an on/off/inverter circuit.

[0122] This circuit includes two temperature sensors R2 and R3 in a(halo bridge circuit connected to the positive input of a comparator.The other half of the bridge circuit is formed by the resistances R5 andR6.

[0123] By way of the on/off-inverter circuit, the controller is capableof switching the electric switches Q1 and Q2 as shown in FIG. 20, but isalso capable of simultaneously switching off both switches.

[0124] In the “off” state of both switches, the controller sets thecircuit to a calibration state. At that moment, the device no longeracts as a flow meter. In this situation, neither of the objects willdissippate anymore, nor assume the temperature of the medium. After acertain time delay, in which the objects have physically acquired anequal temperature, the controller, using an analogue/digital converterand a digital/analogue converter circuit, may determine thecalibration-offset value and store it an own memory.

[0125] In the calibration phase, the output of the comparator ismonitored by an analogue/digital converter.

[0126] The digital/analogue converter injects a (small) offset flow intothe negative input of the comparator. Using the digital/analogueconverter, the controller controls the offset flow referred to earlierin such a manner that the comparator just inverts. The value of theoffset flow associated with this just inversion of the comparator is thecalibration-offset value referred to in the invention.

[0127] An embodiment of the invention is shown in FIG. 25. Here, totemperature-dependent resistances are used as objects. By applying theelectric flows I₁,I₂ and measuring the resulting voltages V₁,V₂, thecontroller knows the powers (P_(gen1)=I₁*V₁) and (P_(gen2)=I₂*V₂)dissipated and generated in the two objects. Also, the controller knowsthe temperature of both objects by simultaneously determining V₁/I₁=R₁and V₂/I₂=R₂.

[0128] Drawn is a digital (possibly proportional) controller having twoanalogue/digital converters and two digital/analogue converters, but ofcourse the controller may also be analogously designed, the convertersbeing omitted.

[0129] Another embodiment of the invention is shown in FIG. 26. Here,the two objects consist of resistances on the one hand and thermocoupletemperature sensors on the other.

[0130] Just as in FIG. 25, use is made here of resistances asdissipators for generating heat in the objects but, contrary to theembodiment of FIG. 25, there is made use of a thermopile as a sensor forthe temperature difference between the two objects.

[0131] The power dissipated in the two resistances is controlled andcalculated in the same way as for the embodiment of FIG. 25.

[0132] The thermopile directly generates a difference signal directlyproportional to T₁−T₂: ΔT=constant*(T₁−T₂).

[0133] Both with the embodiment as shown in FIG. 25 and with theembodiment in FIG. 26, use is made of two objects whose mutualtemperature difference is adjusted to zero.

[0134] In addition, in FIG. 27 there is given an embodiment of themedium flow meter in which two pairs of objects according to theinvention are controlled.

[0135] In this embodiment of the flow-measuring device, both thetemperature difference between the objects O₁ and O₂ is adjusted to zeroand the temperature difference between the objects O₃ and O₄ is adjustedto zero.

[0136] Both pairs of objects are adjusted byway of the CTA method(distribution as in FIG. 17). Here, the target temperature of bothobjects O₁ and O₂ lies at a fixed value over the value of the targettemperature of the objects O₃ and O₄.

[0137] With the constant temperature of the objects O₃ and O₄, there maypossibly be prevented asymmetric outside influences from the pair ofobjects O₁,O₂.

[0138] In an embodiment of the invention, such as the one shown in FIG.28, there are included n/2 pairs of objects in a flow-measuring device.The temperature of all pairs of objects is adjusted, by way of the CTAmethod (see FIG. 17, inter alia), to a constant temperature by way ofjust as many controllers controlling the dissipation powers.

[0139] The resulting temperature distribution as a function of theposition for two different flows is outlined in FIG. 28 (middle). Therequired total powers (always P₁+P₂ per controller) are shown in FIG. 28(bottom). In this embodiment, there is built in a redundancy, so that aflow-measuring system consisting of n/2 combinations of the invention inthe event of down time of one of the n/2 object-pair controller systemsis designed in such a manner that it can continue to operate.

[0140] In FIG. 29, there is indicated how an object O₁ may beconstructed in such a manner that the object completely encloses the(flowing) medium. Also, the length I of the object O₁ in the directionof the tubular construction is chosen is such a manner that thetemperature of the medium at the position of O₁ becomes equal to theobject temperature T₁ measured in this situation, use is made of acompletely developed temperature profile in the direction perpendicularto the tubular construction.

[0141] In FIG. 30, two objects are mounted around the tubularconstruction. The length of the objects is such that there comes intoexistence a completely developed temperature profile (cf. FIG. 25 andFIG. 26).

[0142] In FIG. 31 (top) there is given a cross section of two objects ina flowing medium. In FIG. 31 (bottom), there is indicated in which waytwo tubular objects may be constructed in the direction perpendicular toanother tubular construction enclosing the medium.

[0143] In FIG. 32, there is given a cross section of an embodiment inwhich the two objects are each configured on a kind of probeconstruction.

[0144] In FIG. 33, there is given an embodiment in which the two objectsare separated from the medium by a (thin) partition. Said partition maybe made of, e.g., stainless steel.

[0145] Said embodiment of two objects, as given in FIG. 33, may beincluded in the sidewall of a tubular construction, as given in FIG. 35.The embodiment of two objects as given in FIG. 33 may be included in thefront side of a “probe” construction, i.e., in a tubular construction asgiven in FIG. 34.

[0146] In FIG. 37, there is given a plan view of the embodiment. Theouter dimensions are 1 millimeter by 2 millimeters. There are twoobjects, consisting of a beam, which are suspended freely above adepression etched out. Each object or beam supports two resistances. Thelength of the objects in the plan view of FIG. 37 is 1 mm.

[0147] In FIG. 36, there is drawn a cross section of said embodiment ofthe two objects. The dimensions of the two objects in the cross sectionare 1 micrometer thick and 40 micrometers long in the flow direction.

[0148] A cross section of an embodiment of the invention is shown inFIG. 38. Four resistance tracks on a synthetic carrier, two by two forman object which may be controlled as explained earlier (with FIG. 14).

[0149] The figures FIGS. 39a and 39 b show substantially equalstructures, use being made of a wide depression over which there areextended beams. On each beam, there lie one or more resistances.

REFERENCES

[0150] [R1] H. H. Bruun, “Hot wire anemometry, principles and signalanalysis”, Oxford University Press, Oxford, 1995.

[0151] [R2] U.S. Pat. No. 4,651,564, R. G. Johnson and R. E. Higashi,“Semiconductor device”.

[0152] [R3] T. S. J. Lammerink et al., “Micro-Liquid Flow Sensor”,Sensors and Actuators A, 37-38, (1993), pp. 45-50.

[0153] [R4] H. J. Verhoeven, “Smart Thermal Flow Sensors”, Thesis, 1995,Delft University.

[0154] [R5] U.S. Pat. No. 5,036,701, F. van der Graaf, “Mass-flow meterwith temperature sensors”.

[0155] [R6] H. E. de Bree et al., “The microflown, a novel devicemeasuring acoustical flows”, Sensors and Actuators, S&A54/1-3, pp.552-557.

[0156] [R7] H- E. de Bree, “The Microflown”, Thesis, 1997, ISBN9036509262, Twente University, Enschede.

[0157] [R8] A. F. P. van Putten: “A constant voltage constant currentWheatstone bridge configuration”, Sensors and Actuators, 13 (1988), pp.103-115.

[0158] [R9] U.S. Pat. No. 4,548,077, A. F. P. van Putten, “Ambienttemperature compensated double bridge anemometer”.

[0159] [R10] U.S. Pat. No. 5,064,296, J. H. Huijsing and F. R. Riedijk,“Integrated semiconductor circuit for thermal measurements”.

1. Device for measuring the flow of a medium, e.g., a gaseous medium ora fluid medium, which device is based on measuring and affecting thetemperature distribution of a medium flowing along, which flow sensorcomprises two objects, each comprising a heating element and a way of acontrol loop, to the value of zero, the asymmetry of the power supply tothe objects in order to comply with the aforesaid criterion, accordingto which the temperature difference must be zero, being representativeof the value of het medium flow to be measured, taking into accountparameters associated with the medium, such as its density and specifiche at.
 2. Device according to claim 1, comprising several pairs ofobjects having, for each pair, controlled temperature differences ofzero, the distribution of the power supply among object-heating elementsbeing representative of the flow.
 3. Device according to claim 1 or 2,in which use is made of a constant total sum of the object powers. 4.Device according to claim 1 or 2, in which use is made of a constanttemperature of said objects with respect to the environment.
 5. Deviceaccording to any of the preceding claims, in which the heating elementand the temperature sensor are integrated into one object and the sensoris capable of optionally operating on the basis of twotemperature-dependent resistances having a negative or positivetemperature coefficient.
 6. Device according to claim 5, in which thesimultaneous heating of an object and the measurement of the temperatureof said objects is capable of being carried out using the ω→3ω methodknown per se.
 7. Device according to any of the preceding claims, inwhich the flow measurement is carried out for more than one dimension.8. Device according to any of the preceding claims, in which themeasurement of the object-temperature differences is carried out using athermocouple or thermopile.
 9. Device according to any of the precedingclaims, in which a temperature sensor is based on a material having atemperature-dependent resistance, such as a suitable metal or alloy, asemiconductor, a material having a PTC or NTC [=positive or negativetemperature coefficient], or the like.
 10. Device according to any ofthe preceding claims, in which the device is implemented as asemiconductor chip.
 11. Device according to claim 10, in which the chipis based on silicon.
 12. Device according to any of the precedingclaims, in which the device is bidirectionally sensitive.
 13. Deviceaccording to any of the preceding claims, comprising self-diagnosisprovisions, the stability of the system being capable of beingdetermined on the basis of assessment of the control loop, and anoperational failure of a temperature sensor or dissipator being capableof being detected.